Histological and ultrastructural studies revealed that the average size of elastic fibers in thickened endocardium was much larger in congenital than in acquired endocardial fibroelastosis. BIBLIOGRAPHIC REFERENCES: Fishbein, M.C., Ferrans, V.J. and Roberts, W.C.: Histologic and ultrastructural features of primary and secondary endocardial fibroelastosis. Arch Pathol Lab Med 101: 49-54, 1977.